Colonies of Asian species of Mus established in this Laboratory were used as a basis for the study of genetic variability of murine endogenous retroviruses and the genetic elements which affect their expression and/or involvement in neoplastic disease. Four different classes of endogenous retroviruses have been isolated from Asian Mus species. These include two distinct classes of type-C virus (designated C-I and C-II), a novel class of viruses (M432 group) and milkborne type-B viruses. The type-C and M432 group of viruses were obtained either from cultured cell lines which spontaneously began producing virions or by induction of tissue culture cells with pyrimidine analogues followed by cocultivation with sensitive indicator cell lines. The major internal viral protein (p30) of the C-I class competes in a woolly monkey - gibbon ape group-specific radioimmunoassay, whereas, the respective protein of the C-II class shows no competition.